A driving force controlling apparatus for a vehicle including a motor for driving wheels other than driving wheels driven by an engine, with electric power from an electric generator is known conventionally. In an AC motor electric 4WD including an AC motor existing on a non-engine shaft and driven by electric power from an alternator, a request is made of the alternator to generate electric power necessary for the motor. For this purpose, there is a demand for a technique for causing the alternator to stably output the requested voltage to enable the motor to operate stably.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-206040    Patent Document 2: Japanese Patent Application Laid-open No. 2006-306144    Patent Document 3: Japanese Patent Application Laid-open No. 2006-296132    Patent Document 4: Japanese Patent Application Laid-open No. 2006-315660